PVC films are typically heavily loaded with plasticizer to provide durability and resistance against cracking. It has long been known that PVC will yellow due to exposure to UV light that are believed to generate free radicals in the polymer. The incorporation of UV stabilizers into PVC allows the production of stabilized vinyl films to have substantially reduced tendency toward yellowing from UV light exposure. In the recent years, however, there has been a general trend in certain applications to coat vinyl films with UV-cured top coats to increase the gloss, abrasion resistance and gloss retention. Applications where vinyl is coated with a UV-cured top coat include decals, labels and vinyl flooring.
Certain tests support a belief that the use of UV stabilizers is sufficient to stabilize UV-cured top-coated vinyl against yellowing. For instance, samples of freshly-prepared, UV-cured top-coated vinyl films that are exposed to artificial accelerated UV light sources, e.g. Atlas Corporation's Xenon weatherometer, show significant resistance toward yellowing.
I have discovered that if the same UV-cured top-coated, UV-stabilized vinyl films are allowed to age, e.g. by exposure to time or heat, the vinyl films will yellow significantly with exposure to UV light. Interestingly enough, the vinyl layer and not the UV-cured top coating appears to be the layer that is susceptible to yellow under aging conditions. UV-cured top coatings include acrylate monomer and urethane-acrylate formulations containing a UV activated polymerization initiator species. UV initiator species are typically molecules designed to absorb UV light and break down to generate radicals that can initiate the polymerization of the monomers. It is known by practitioners in the UV-coating industry that the curing process does not consume all of the UV initiator species that are present in coating formulations. I believe the cause of this delayed yellowing phenomena upon aging is due to the plasticizer in the vinyl migrating into contact with the UV-cured top coating, thereby providing a path for the unreacted initiator migrate to the vinyl layer. In the vinyl layer, the initiator can, when exposed to UV light, generate radicals which can degrade the vinyl or additives such as plasticizers, manifested as yellowing.